Starting electrode for pool tubes



H. E. ROWE STARTING ELECTRODE FOR POOL TUBES May 23, 1939.

Filed April 16, 1936 Inventor Hobart E. Rowe b x MW y His A D/AME TE)? INCHES ttorney.

Patented MayZS, i939 I r V UNITED STATES PATENT OFFICE STARTING ELECTRODE FOR POOL TUBES Hobart E. Rowe, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application April 16, 1936, Serial No. 74,653

1 Claim. 250-275) I The present invention relates to pool type disternal connection is brought out from the charge devices in which an arc discharge is conmercury by means of a leading-in conductor 6, trolled by a starting electrode projecting below suitably sealed in the envelope.

the surface of the pool. More particularly, my The main anode 7 of the device may take the 6 invention provides a starting electrode of an imform of a cylinder of refractory material such proved type. as graphite or metal and is mounted in place by In the prior vapor electric devices, more means of a rigid rod 8 which is secured to the especially those which contain an excess of vaporstem 2 opposite from the cathode. A leading-in producing material in the form of a liquid which conductor 9 is connected to the rod 8.

10 serves as a pool cathode, a great deal of power For starting the are between the main elec- 10 is Wasted in initiating the discharge. The starttrodes and more particularly. for producing a ing current is usually low but thevoltage necescathode'spot on the mercury, there is provided a sary to produce a hot spot on the pool cathode starting electrode ill which fixedly projects beis often high and excessive. I have found in aclow the surface of the mercury 5 during opera.-

cordance with my invention, that the excessive tion. As shown more clearly in Fig. 2, this start-' starting voltage is due, at least in part, to the ing electrode is mounted in position by means of I configuration of that portion of the starting eleca cylindrical support member II having a trode which is immersed in the pool. counterbored portion which receives the elec- One of the objects of the present invention is trode.

to provide vapor electric apparatus in'which the The starting electrode is preferably composed 20 power required for initiating the discharge is a of a semi-conducting material such as tungsten minimum, and more particularly, to reduce the carbide or silicon carbide, suitably combined with starting voltage of those devices which employ clay or a similar binder, and takes the form of a starting electrodes of semi-c0nducting material cylindrical member which for reasons given which permanently project into a vapor-producpresently, has a taper over that portion of its 26 ing liquid. In carrying out this object, I modify length which comes into contact with the the configuration of the starting electrode. mercury.

Whereas prior starting electrodes generally take The material found most suitable for this purthe form of a cylinder or rod of a uniform and pose is composed of 55 per cent silica, flint or relatively small diameter, the electrode as imquartzite in ground or finely divided form, 25 per 30 proved in accordance with my invention is made cent clay, and 20 per cent graphite, thoroughly very large at its upper end and is tapered to a mixed in the presence of a small quantity of very small diameter, even to a point, at the end water. It will, of course, be understood that satisu which extends into the pool. Theinvention will factory starting electrodes can be made of other be more clearly understood when reference is moldable materials and in difierent proportions 35 i made to the following description and the acthan those stated. The silica or similar material companying drawing in which Fig. 1 is a diagramis used as a filler, suitable from the stability matic view of typical starting and load cirof operation standpoint, as well as cheap in cost,

cults, also of a vapor electric tube which conto render the mixture less critical to small tains a starting electrode of the improved. type; changes in composition of the other constituents. 40

Fig. 2 is an enlarged view in cross-section of the The clay constitutes a dispersing agent for the starting electrode shown in Fig. 1; Fig. 3 is a graphite and as abinder to bond the mixtureinto graphicalrepresentation of a relationship which a suitable material. The graphite provides the .1 explains the nature of my invention; and Fig. 4 electrical conducting medium. The mixture shows a modified form of starting electrode. forms a paste and may be either cast in a mold I Referring more particularly to Fig. l, numeral or extruded through a die to provide a rod,

l designates an evacuated envelope which terpreferably of cylindrical shape. These rods are minates at each end in stem portions 2 of rethen packed in graphite powder, placed in a [duced size. Projecting from the side of the enhydrogen furnace, and raised in temperature to 5 0 velope I, there is a tubular extension 3 which is 1250 or 1300 C. They are held at this tempera- 0 l provided with a reentrant stem 4. In the lower ture approximately 30 minutes to ensure complete end of the envelope, is collected a quantity of sintering. A longer or shorter time for sintering .vapor-producing liquid .5 such as mercury, could be used if precision measurements of tem- Wgallium, or. tin which serves as the cathode of perature are made. After firing, the parts are 15 5}.the main discharge through the device. An exallowed to cool in the furnace to room tempera- III ture. If fired at a temperature in excess of 1300 C., an undesired glazed surface of silicon carbide results. On the other hand, if the rods are fired at a temperature lower than 1250 0., the sintering is incomplete and the clay renders the material soft and fragile. The material made in this manner is found to be reasonably free from gas during the regular exhaust of the tube. The material is also unaffected by the normal operation of the tube. These rods may be cut into sections of proper length, the ends of which are tapered, preferably to a point by grinding,

to a close tolerance in order to ensure reproducibility of the operationof-the tube.

The support member ll ispreferably held in a vertical position in such a manner that the starting electrode l extends in a direction substantially normal to the level of the mercury when the tube l is in its operating vertical position. The support'member may be either swaged or Welded to a rigid rod IZhaving a transversely extending portion which is sealed in the stem 4. If *desired; a pant-leg I3; maybe provided around the red at the placewhere it enters the stem 4 to prevent condensation of the mercury and hence reduce electricalleakage'at this point. A cap I' l is secured toithe outside surface of the glass arm3;and"a"connection "is made between this cap member and therod l2.

The external circuit for the main electrodes includes a source of" alternating current, as indicaftedfand' a load generically represented by aresi'stance i5 which is"preferably shunted by a smoothing condenser l 5, This circuit is connectedt'o the'leading 'in conductors B and 9.

The's'tartingelectrode circuit includes a thermioriic device if and a current limiting resistance l8, thiscircuit being connected between the cap M and the leading-in conductor 9. The thermionic device includes a filamentary cathode "l',"heated in any suitable manner, and an anode 20, thejfilanfentf beingbonnected directly to the starting electrode so that the device permits currenttofiowtothe starting electrode only during that timevvh'en the main anbde is positive with respect to'themairi cathode. If desired, a grid 2| may be providedbetween'the cathode and the anode of the thermionic device in order to'contro'l the time at'whiclithe starting electrode 'will be energized' A shunticircuitis provided between the 'gridandthe cathode, this circuit including a battery"22 and"'a current limiting resistance 23, thebattery being so poled, as indicated, to provide acons'tant negative bias on the grid. This jbias'is so high that no'current normally flows Tbetween the anode and cathode of the thermionic device, and to permit current to flow at the desiredt'ime", a shunt circuit is' provided between theg'rid'an'd the anode through a current limiting resistance 24 and a'contrjol switch 25. When theswit'ch 25 isclosedfthe negative bias on the grid is dissipated and'the potential of the latter 'approachesthatof' the positively charged anode 2i), thusproviding a path of relatively low impedance betweenthe'anode 2E! and oathodel9.

A's explainedhereinbefore,'the object of the present invention is to reducethe necessary power and more particularly, the voltage appliedto thev starting electrode for initiatingthe arc discharge. Referring now to Fig, 3, which is a curve plotted against the starting current in amperes as ordinate and the diameter of the starting electrode at theline of contact with the, mercury surface, asabsoissa,itwillbe noted that the startn current s a il ed es as the si e. o the starting electrode is decreased. Consequently, in order to keep the starting current as low as possible, it is desirable to have the diameter of the starting electrode at the line of contact with the mercury surface as small as possible, even as small as a point. On the other hand, when the starting electrode is of very small diameter, the voltage drop in the electrode becomes considerable. It is apparent that because of its semi-conducting character, the starting electrode acts as a resistance element in series with the starting circuit, the value of which changes in accordance with the length of electrode between the surface of themercury andthe metal support member. Under these conditions, if the voltage drop in the body of the starting electrode is high, the potential difference between the starting electrode and the mercury at the place of contact might be too low to initiate adischarge, unless impractically high voltages were applied to the starting electrode.

However, in accordance with my invention, the advantage of a starting electrode of, relatively small diameter at the line of contact with the mercury and its desirable property of requiring a low starting'current are obtained and at the same is ordinarily used and tapering the electrode to a,

point, or substantially so, along that portion of its surface which actually is in contact with the mercury or is likely to be in contact when the level of the mercury changes due to movements of the mercury surface during operation.

For the purpose of my invention, 1. e. to obtain as small a voltage loss in the starting electrode as possible, and'hence, provide an electrode'which requires, the minimum. starting potential, the taper is preferably straight, as'in'dicated in Fig. 2. The present invention, therefore, contemplatesastarting electrode for pool ,types of, tubes in whichthe upperor shank end of the electrode is made considerably larger than the correspondinglpo rtionof the prior art starting electrodes and notwithstanding this greater volume of electrode material, the starting current is still maintained at a relatively 'low valueby reducing the diameter, or in any other manner decreasing the cross-sectional area of the electrode at the line of contact with the mercury, even to'a point, although under practical conditions, the electrode need notbe tapered to such a degree. The degree of taper may be determined by c gperiment based on operating conditions. The diameter of the upper Orshank end of the starting electrode of the electrode at the line of contact with the mercury affect the current starting conditions. In general, the larger the diameteratthe upper end of the starting electrode; the less voltage drop there will be in'this member and hence; the less starting voltage willbenecessary m initiate a discharge.

I have u ce iullr sedqstarting electr d s composed of variousicombinationsof materials includingtungsten carbide. and clay, as well. as

those set forth hereinbefore, formed into a rod of i .25 inch diameter which was tapered over a length of .5 inch to substantially a point.

In addition to providing a smaller voltage drop through the body of the electrode, the increased size of the upper end of thestarting electrode is of marked advantage in ofiering a better support at this end. A more rigid joint is provided between the starting electrode and the metal supporting member due to the larger area in contact, and the electrical contact between these members is corresponding enhanced.

i In addition to offering a reduced starting voltage, the over-sized tapered form of starting electrode actually assists in other ways to produce the necessary hot spot on the mercury pool. It is Well known that the meniscus of the mercury pool at the place of contact with the starting electrode tends to bend downwardly and outwardly, due to capillary attraction or otherwise, and in general, to take on a convex shape at this position. This phenomenon is shown in Fig. 2 and is indicated at 26. The taper of the electrode might therefore be designed, if desired, to match as far as possible, and without departing from astraight taper, the convexity of the mercury surface at this position so as, in efiect, to provide a greater area of contact between the mercury and the starting electrode. The increased area of contact appears to encourage the production of minute sparks in large numbers between the mercury and the starting electrode and thus further tends to facilitate starting under reduced voltage and current conditions.

While I have explained my invention more especially in connection with a tapered starting electrode, the invention is not limited to an electrode of this configuration. As stated hereinbefore, the invention contemplates a starting electrode in which the upper or shank end is over size and is made considerably larger than the starting electrodes of the prior art. The electrode is then tapered from the over size portion to such a size at the line of contact with the mercury that the starting current is reasonably low. This taper may be gradual or may constitute an abrupt change in diameter. Thus the starting electrode may be constituted of any number of portions or sections having diiferent and constant diameters over each portion or section so that the electrode takes the form of a stepped-down diameter configuration, approaching a point in size. An electrode of this type is shown in Fig. 4, the uppermost portion 21 of which is of a relatively large diameter so as to produce little or no voltage drop therein and thus bring to the lowermost or immersed portion 28 of the electrode, substantially all of the available voltage by which to start the arc. It will be noted that the diameter of the portion 28 is sufliciently small to minimize the starting current and the electrode as a whole tapers down substantially to a point. Hence, as in the case of the starting electrode shown in Fig. 2, the electrode illustrated in Fig. 4 offers the double advantage of requiring the minimum starting voltage and starting current for initiating an arc discharge.

What I claim as new and desire to secure by Letters Patent of the United States is:

An electron discharge device comprising a pooltype cathode, an anode, and a starting electrode in the form of a semi-conducting member having a portion thereof immersed in the cathode material during the normal operation of the device, said member. being constituted of a plurality of cylindrical sections of varying diameters joined in end-to-end relationship, the sections being arranged in order of decreasing diameters so thesection of least diameter forms the tip of the saidimmersed portion of the member.

HOBART E. ROWE. 

